Codex/fix mobile back highlight aggregate

[Just-Insane]

Description

Fixes #

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings

AI disclosure:

• Partially AI assisted (clarify which code was AI assisted and briefly explain what it does).
• Fully AI generated (explain what all the generated code does in moderate detail).

[github-actions]

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Status	Preview URL	Commit	Alias	Updated (UTC)
✅ Deployment successful!	https://pr-41-sable.justin-tech.workers.dev	a813576	pr-41	Fri, 12 Jun 2026 15:03:12 GMT

[Copilot]

Pull request overview

This PR updates the mobile Room view header’s back-button highlight badge logic so it reflects an aggregated highlight count across message rooms, rather than deriving the count via route/category-specific room lists.

Changes:

• Replace route matching + per-scope unread aggregation with a single aggregation over selected “message rooms” (non-space rooms).
• Remove now-unused router/path imports and room-list scope hooks used only for the previous back-target calculation.